[LyX/master] Revert "Support the mathbbm font."

[Thibaut Cuvelier]

commit d75ff9931d978ff7d105a3d1d49c87ba27f09d81
Author: Thibaut Cuvelier <tcuvelier at lyx.org>
Date:   Tue Jul 28 18:01:38 2020 +0200

    Revert "Support the mathbbm font."
    
    This reverts commit e709a6626e731b460a98ea34d762a776de90b54c.

 lib/fonts/bbm.sfd                  | 6448 ------------------------------------
 lib/fonts/bbm.ttf                  |  Bin 19696 -> 0 bytes
 lib/symbols                        |    1 -
 src/FontEnums.h                    |    2 -
 src/FontInfo.cpp                   |    1 -
 src/LaTeXFeatures.cpp              |    3 +-
 src/frontends/qt/GuiFontLoader.cpp |    4 +-
 src/mathed/InsetMathFont.cpp       |    2 -
 src/mathed/MathSupport.cpp         |    2 -
 src/output_docbook.cpp             |   31 +-
 10 files changed, 14 insertions(+), 6480 deletions(-)